GSH transferase

Balance of Activatlon/DetoxifIcatlon Reactions. The activated intermediates or reactive fragments appear to be carbamoyl sulfoxides or mono-, dl- and trlchloroacroleins, all of which are relatively unstable compounds. The carbamoyl sulfoxides are rapidly detoxified by reaction with GSH, involving catalysis by a GSH -transferase in the case of -alkyl and -benzyl thiocarba-mate sulfoxides ( -5, 24) but probably not with -chloro-... 

GSH may also be coupled to electrophilic reaction intermediates nonenzymatically or by GSH transferase (GST)-catalyzed reactions. Many different types of substrates will undergo GSH conjugation, including epoxides, halogenated compounds, aromatic nitro compounds, and many others. In these reactions, GSH can interact with an electrophilic carbon or heteroatom (O, N, and S) [35]. One such substrate is a reactive metabolite of acetaminophen (APAP), N-acetyl-p-benzoquinonimine (NAPQI), which will readily form a GSH conjugate (Scheme 3.2). Other examples of Phase II bioactivation reactions that lead to toxic endpoints are shown in Table 3.1. 

Metabolic activation. Although the kidney does not contain as much cytochromes P-450 as the liver, there is sufficient activity to be responsible for metabolic activation, and other oxidative enzymes such as those of the prostaglandin synthetase system are also present. Such metabolic activation may underlie the renal toxicity of chloroform and paracetamol (see chap. 7). Other enzymes such as C-S lyase and GSH transferase may also be involved in the activation of compounds such as hexachlorobutadiene (see chap. 7). In some cases, hepatic metabolism may be involved followed by transport to the kidney and subsequent toxicity. 

As already discussed in chapter 4, reactive intermediates can react with reduced GSH either by a direct chemical reaction or by a GSH transferase-mediated reaction. If excessive, these reactions can deplete the cellular GSH. Also, reactive metabolites can oxidize GSH and other thiol groups such as those in proteins and thereby cause a change in thiol status. When the rate of oxidation of GSH exceeds the capacity of GSH reductase, then oxidized glutathione (GSSG) is actively transported out of the cell and thereby lost. Thus, reduced GSH may be removed reversibly by oxidation or formation of mixed disulfides with proteins and irreversibly by conjugation or loss of the oxidized form from the cell. Thus, after exposure of cells to quinones such as menadione, which cause oxidative stress, GSH conjugates, mixed disulfides, and GSSG are formed, all of which will reduce the cellular GSH level. 

TCDD binds to the AhR and initiates a number of responses as well as induction of CYP1A1, including induction of UDP-glucuronosyl transferases, GSH transferases, and stimulation of apoptosis in thymocytes, resulting in thymic atrophy. 

Several other cellular target proteins for the reactive metabolite of paracetamol have also been detected and identified, namely, formyl tetrahydrofolate dehydrogenase, glyceralde-hyde-3-phosphate dehydrogenase (GAPDH), and GSH transferase, all cytosolic proteins. 

As has been stated before, oxirane derivatives are formed as intermediates during metabolic oxidations at carbon-carbon double bonds. These epoxides (arene oxides) undergo spontaneous isomerization to phenols, or enzymic hydration via epoxide hydrase to trans- dihydrodiols, or reaction with reduced glutathione (GSH) via specific GSH-transferases to the corresponding conjugates (Scheme 11), which eventually appear in urine... 

Meyer DJ, Thomas M (1995) Characterization of rat spleen prostaglandin H D-isomerase as a sigma-class GSH transferase. Biochem J 311 739-742... 

Many halogenated hydrocarbons are substrates for GSH-transferase-catalyzed nucleophilic substitution reactions that produce S-substituted glutathione (GSH) derivatives. These are normal SN2 displacements of halide with thiolate anion that occur with inver-... 

Many polyhalogenated alkenes are potent nephrotoxins. 2-Chloro-l, 1,2-trifluoroethylene (CTFE), trichloroethylene (TCE) and tetrafluoroethylene (TFE) are notable examples. The initial step in bioactivation of such compounds is the GSH-trans-ferase-catalyzed addition or addition-elimination of GSH. For example, in the liver, the action of GSH- transferase converts CTFE to S-(2-chloro- l,l,2-trifluoroethyl)glu-tathione (CTFG), TCE to S-(l,2-dichlorovinylglutathione (DCVG) and TFE to S-... 

Metabolic activation of chemical carcinogens by cytochrome P-450 is well-documented. In addition to the Phase 1 enzymes, Phase 2 enzymes such as GSH transferases can also participate in metabolically activating or inactivating (detoxification) chemical carcinogens. Human genetic polymorphisms have been characterized in some Phase 1 and Phase 2 genes (see Chapters 11 and 13). Such polymorphisms could alter an individual s susceptibility to cancer induced by certain chemical... 

Electrophiles not only complex with macromolecular nucleophilic sites, but also complex with those of small molecules. The most important of these is the -SH group of GSH. Conjugation of electrophiles with GSH is an important detoxication reaction catalyzed by GSH transferases (see Chapter 12). Although GSH concentration in hepatocytes is high relative to other tissues (4-8mM), depletion of GSH can occur upon acute exposure to protoxicants whose electrophilic metabolites... 

Apart from stereoselective bioactivation, stereoselective bioinactivation may be important too from a toxicological point of view. For instance, in contrast to the basic (a-e), the human near neutral ( i,) and acidic (tt) classes of GSH transferases were efficient in the conjugation of benz[aj-pyrene-7,8-diol-9,10-epoxides and, in particular, the carcinogenic (+)-anti-enantiomer (and other +)-flwh-enantiomers of the b -region diol-epoxides of ber z[a]anthracene and chrysene) (Jernstrom et al 1985 Robertson et al., 1986). 

In summary, GSH transferase isoenzymes operate highly stereoselec-tively and this may have important consequences for the detoxification or toxification of chiral or pro-chiral substrates,... 

Robertson, I. G. C., Guthenberg, C., Maimervik, B., and Jemstrom, B. (1986). Differences in Stereoselectivity and Catalytic Efficiency of Three Human GSH-Transferases in the Conjugation of GSH with 7B,8 -Dihydroxy-9a,10a-oxy-7,8,9,10-tetrahydrobenzola]pyrene. Cancer Res., 46 2220-2224. 

GSH transferases are inducible enzymes with overlapping substrate specificity [73]. They are also found in renal cells as cytosolic enzymes or as membrane-bound microsomal transferases. GSH conjugates are usually less toxic than their parent compounds and are readily excreted in the bile and in the urine as their correspond-... 

Many tissues are capable, at least vitro, of conjugation of xenobiotics with GSH (l)j the liver is assumed to be the most active, especially with Ingested xenobiotics, however GSH transferase activity has been detected in the Intestinal mucosa (6). The interorgan translocation of GSH conjugates formed in the intestinal... 

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